The present invention relates to increasing the sensitivity of a gas detector.
There is an unending need to improve the sensitivity of analytical methods and apparatus. In the field of gas detection, for example, improving sensitivity can improve accuracy, the ability to detect trace amounts of a gas, the safety of a system, and process quality.
Gas detectors are often used to detect a gas of interest under various conditions and in a variety of environments. Many times it is also necessary to continuously monitor for the presence and amount of a gas in an environment. Often it is desirable that the detector measure the concentration of a gas in an accurate and timely manner
Gas detectors often include a source of radiation, a sample chamber and a detector. Various attempts have been made to increase the sensitivity of gas detectors including increasing the distance between the source of radiation and the detector, which distance is referred to as the “optical path length.” Increasing this distance can increase the amount of sample measured, and therefore the sensitivity of the analysis. Theoretically, a larger sample volume will contain a larger number of gas particles, which in turn can increase the sensitivity of the detector.
One way in which the optical path length has been increased has been through the use of reflective mirrors. The use of mirrors can be undesirable, however, because mirrors tend to corrode and accumulate residue over time. As the residue builds up, the amount of radiation reflected from the mirror decreases, which contributes to a decrease in the useful life of the detector. This is particularly true when vapors, such as water vapors, are present in the sample being measured.
Others have attempted to increase the optical path length by physically increasing the length of the sample chamber, which tends to increase the length of the apparatus as a whole. In addition to undesirably increasing the overall bulk of the apparatus, such an arrangement tends to increase the radiation scattering due to the increased distance between the excitation source and the detector.
Increasing the length of the optical path in a gas detector does not necessarily provide a corresponding increase in the amount of energy capable of being measured. To the contrary, increasing the path length often results in diminishing returns in terms of energy. FIG. 1 illustrates the relationship between optical path length and absorption. The increase in absorbance is exponential relative to an increase in optical path length. As the optical path length gets larger, an incremental increase in path length provides a relatively small increase in absorbance, rendering the increase in path length less efficient in terms of absorption. Thus, incrementally increasing the optical path length of the detector becomes an increasingly less efficient means for increasing the amount of measurable energy.
Methods and apparatus for improving the sensitivity of gas detectors are needed. Increasing the sample size and hence sensitivity of gas detectors is particularly desired, particularly where it is desirable to quantify the concentration of the gas of interest.